The invention relates to a process for manufacturing a metal ceramic substratum for use as a substrate in electrical or electronic circuits or components.
The metallization required for production for conductor tracks, connections, etc. to be produced on a ceramic, such as, an aluminum-oxide-ceramic, can be accomplished with the DCB process (Direct Copper Bond Technology). This can be accomplished by using metallizationxe2x80x94producing metal foils or copper foils or metal/copper sheets, which have on their upper surface sides, a layer or coating (sintering layer) made of a chemical compound of the metal and a reactive gas, preferably oxygen.
In this procedure, as described for example, in the patent specification for U.S. Pat. No. 3,744,120, or German Patent No. DE 2,319,854, this layer or coating (sintering layer) builds up an eutectic mixture, having a fusion temperature lying below the fusion temperature of the metal (such as copper), so that by placing the foil on the ceramic and by heating up all the layers, these layers can be joined with one another. Fusion of the metal or copper only takes place in the region of the fusion layer or oxide layer.
This DCB process includes the following procedural steps:
Oxidation of a copper-foil in such manner, that there results an uniform copper oxide layer.
Layering of the copper foil on the ceramic layer.
Heating up of the composite block to a processxe2x80x94temperature lying between 1065xc2x0 C. and 1085xc2x0 C., such as about 1071xc2x0 C.
Cooling down to room temperature.
An arched substratum and its manufacture (see EP 0,279,601) are known. In the known case, the metal layer on the concavely arched upper side of the ceramic layer has a distinctly larger thickness than the convexly-arched bottom side of the substratum. This structure is necessary in the known case, because the arching of the substratum is achieved during the cooling down period after the DCB process, with which the two metallizations are to be fixed upon the ceramic layer or the ceramic plate. Because the metallization on the upper side has a larger thickness and hence a larger volume, the thermally conditioned tensions, on the upper side, are larger, compared to that on the bottom side, so that the substratum is arched, in the manner mentioned before, during the cooling down period. A disadvantage, here, amongst others, is that this xe2x80x98bi-metalxe2x80x990 effect is reversible, i.e., during heating up there occurs a rebending or re-arching of the substratum. During use of this known substratum for power components, the contact pressure, (as conditioned by the arching) between the bottom side of the substratum and a cooling body, is reduced by increasing the heating, that is, just then, for achieving a particularly low heat resistance, a high contact pressure is necessary between the substratum and the cooling body, it is reduced by heating up of the substratum. Moreover, the bending strength of the known arched substratum is not satisfactory either.
It is an object of the invention to produce a process, with which arched metal-ceramic substratum, can be manufactured without any problem, particularly with the same thickness of metallizations, on the upper and bottom side.
The metal ceramic substratum as manufactured by the process of the present invention, is suitable for an electrical or an electronic power switching network or module and has improved temperature independent heat dissipation, as compared to known arched substrata.
The process, according to the invention, provides for the manufacture of a pre-arched ceramic plate, from a plane or a plane or plane blank plate. In a further step of the process, a metallization is positioned on this pre-arched ceramic plate.
By the curvature of the substratum, or of the ceramic layer, it is possible to bend it elastically in such a manner so that the substratum, on account of the elastic tension of the ceramic layer, lies compactly and securely against a surface of a heat-dissipating metal plate and thereby provides an improved heat dissipation. This occurs even with the use of a cooling paste between the substratum and the metal plate. The curvature of the substrate particularly serves the purpose of an optimum placing of the bottom side or of the bottom metallization of the substratum, for example, against a metal plate and hence, ensures an optimum possible heat transfer between this substratum and a metal plate, building a cooling plate, for example. This improved compounding between the substratum and the metal plate, on account of the pre-arching of the substratum, should remain practically constant, independent of the temperature of the substratum or the metal plate. This demand is no longer fulfilled, when one of the two metal layers has a greater thickness, because the metallization with larger thickness, and hence with larger volume, exercises changing tensions on the relevant surface side of the ceramic layer, depending on temperature, in case of temperature changes. Varying thicknesses of the two metallizations on the upper and bottom side would lead to extra curvature of the substratum, depending on the temperature.
With the degree of curvature, as provided for by the invention, breaking of the ceramic layer is prevented.
In the following, the invention is explained in details, with the help of figures, for a form of construction.